Zafirlukast proven to be clinically safe/effective in the treatment of mild/moderate asthma. Patients having asthma exacerbations w/FEV1 (spirometry <=70%) upon arrival in emergency between ages 18-65 will receive B2-agonists, oral prednisone, and either oral zafirlukast 20 mg,160 mg, or placebo administered in a predetermined randomized manner 30 min. after baseline evaluation is completed and continued emergency therapy. Patients discharged home receive oral steroid, inhaled B2- agonists, asthma medications and either zafirlukast 20 mg orally 2x/dayorplacebo(28 days).